Your layer contains a simple marker symbol with an outline and a halo. The display of this symbol in a basemap layer or map service will potentially differ from your ArcMap display.
- Do nothing. Accept the new symbol drawing as is.
- Alternatively, you can modify the size of the marker symbol halo by editing the symbol.
This warning message is telling you that the halo size of this symbol will be different in a basemap layer or a map service. This is because the drawing engine used by basemap layers and map services takes the outline size of the symbol into account when calculating the halo, and ArcMap does not.